The subphylum Hexapoda, meaning "six legs", is the largest (in terms of number of species) group of arthropods and includes the insects as well as three much smaller groups of arthropods without wings: Collembola, Protura, and Diplura. All of these were called insects before. They form a polyphyletic group because they evolved independently from each other. In other words, hexapoda is divided into the insects and the entognatha. Hexapods are so called because they have a thorax with three pairs of legs. Most other arthropods have more than three pairs of legs.

Alternatively, the Hexapoda could be eliminated as polyphyletic (which it is), the Insecta promoted to sub-phylum, and the minor groups incertae sedis — a Latin term meaning "of uncertain placement". It is used for a taxonomic group when its broader relationships are unknown or undefined.